94 NEW YORK STATE MUSEUM 



in places actually domed the Grenville over itself. Mostly, how- 

 ever, this magma either broke up or tilted masses of Grenville 

 great and small, or broke through it as great off-shoots from the 

 magma, or even more intimately intruded or injected it. The more 

 or less well-developed foliation of both the anorthosite and 

 syenite-granite is regarded as essentially a magmatic flow-structure 

 produced under the pressure conditions of the intrusions. 



At some time after the deposition of the Grenville strata, the 

 whole Adirondack region was elevated well above sea level. 

 Whether this uplift took place prior to the great igneous activity, 

 or during the instrusion of the anorthosite or syenite-granite, or 

 both, is not definitely known, but it is reasonable to think that the 

 same great force which caused the updwelling of one or both these 

 magmas also produced the general uplift of the region. 



Just when the metamorphism of the Greenville strata took 'place 

 is not known, but there is strong evidence that it took place either 

 before or during the great igneous activity and not afterward. 



Following the intrusion of the syenite-granite series, there was a 

 time of minor igneous activity .when the gabbro magma was forced 

 upward into the earth's crust mostly in the form of stocks sharply 

 cutting the older rocks. Intrusions of pegmatite and aplite dikes 

 also took place, some after the gabbro, and some probably before it. 



The general elevation of the region above referred to inaugu- 

 rated a time of profound erosion which lasted at least some mil- 

 lions of years, even well into the Cambrian period of the Paleozoic 

 era. This is proved by the fact that all the rocks above mentioned 

 are now at the surface and exhibit textures and structures which 

 could have been produced only under deep-seated geologic con- 

 ditions, that is many thousands of feet below the surface of the 

 earth. 



After the removal, by erosion, of a great thickness of rock 

 material, the last Prepaleozoic igneous activity of the region took 

 place when the molten diabase was forced into narrow fissures in 

 the earth and cooled in the form of dikes. This diabase is known 

 to cut all the other rocks. That it must have cooled rather near 

 the earth's surface is evidenced by the usually fine-grained to even 

 glassy texture. 



Paleozoic History 



As a result of the vast erosion above mentioned, the whole Adi- 

 rondack area was worn down to near sea level and it presented at 

 most only a very moderate relief. Then, in late Cambrian time, 



